Is what was quoted above the sum total of the "permission text" that comes
with spin? I don't see anything in what you quoted above which gives the
right to modify and/or redistribute spin. It says "Spin is distributed in
source form to encourage research in formal verification, and to help
support a friendly and open exchange of algorithms, ideas, and tools." That
sounds more to me like "we're giving you source to this so you can see how
we did it, which might help you in your endeavours". It says nothing about
modifying the source code to produce an improved version.

There is a more concrete license at the URL in the previous post
(http://cm.bell-labs.com/cm/cs/what/spin/spin_license.html). It was
discussed on this list previously and considered non-free. Though it
wasn't concluded wether spin might be distributed in the non-free
section based on that license. There was the suggestion of asking
uptream for a nicer license to distribute spin under; but there were no
responses about wether that worked or was attempted.